The present invention relates generally to a zoom lens system used with an imaging device, and more particularly to a zoom lens system suitable for use with video cameras, digital still cameras, etc. The present invention also relates to the allowable value required to obtain good images by use of an image pickup element.
So far, many zoom lenses for video cameras have been proposed in the art, as typically disclosed in JP-A's 5-60972, 5-107473 and 6-94997. All examples given therein are directed to a four lens group type zoom lens system comprising, in order from an object side thereof, a first lens group having positive power, a second lens group having negative power, a third lens group having positive power and a fourth lens group having positive power, with each lens group comprising one to three lenses. Referring to the basic role of each lens group in zooming, the second lens group moves flatly to play a chief zooming role, and the fourth lens group acts as a compensator. As in JP-A 6-94997, the third and fourth lens groups are often designed to move together.
In recent years, digital still cameras have attracted public attention as peripheral equipment (image input equipment) for personal computers. Digital still cameras or video cameras are equipment for forming an image on an image sensor such as a CCD and acquiring information in the form of electric signals. Currently available phototaking lenses are all of the telecentric type because for structural reasons of the image sensor, light beams are shaded by the image sensor upon oblique incidence thereon (the so-called shading phenomenon).
Consequently, the same basic type of phototaking lens is used for both cameras. However, there is a large difference between the image qualities required by users. For instance, when video cameras or digital cameras are used for entering images into personal computers, even an image sensor having about 400,000 pixels may be practically acceptable. In applications where printed images are enjoyed, or used in place of silver salt photographs, however, it is required to use an image sensor having more than 1,000,000 pixels or of a so-called mega-pixel class. As a result, the pixel pitch of the sensor becomes small, and the performance, e.g., resolution needed for a phototaking lens used therewith must be increased to an ever higher level, accordingly.
For such a zoom lens system, it is preferable that aberrations produced in each lens group are reduced as much as possible. To this end, it is required that each lens group be made up of some positive and some negative lenses to reduce aberrations produced therein as much as possible. If the amount of chromatic aberrations produced in the first lens group are not reduced as much as possible, then the height of axial marginal light rays is increased to produce a large amount of longitudinal chromatic aberration on the telephoto side. Consequently, the performance of the zoom lens system becomes worse. To make satisfactory correction for chromatic aberrations on the telephoto side, it is thus required to use a doublet or other chromatic aberration corrector element in the first lens group. A typical example of using a chromatic aberration corrector element in the first lens group is a zoom lens system disclosed in JP-A 9-211329 that proposes to use a diffractive optical element (DOE) to make correction for chromatic aberrations.
JP-A 9-211329, mentioned above, proposes to use a diffractive surface in a first or second lens group in a positive/negative/positive/positive four lens group type zoom lens system, thereby achieving a zoom ratio of 5.7. However, this zoom lens system is very unsatisfactory in terms of correction of aberrations irrespective of using the diffractive surface, and so is practically unacceptable.
Applications of diffractive surfaces to various optical systems are shown in "Diffractive optics at Eastman Kodak Company", SPIE, Vol. 2689, pp. 228-254. This article refers primarily to applications of diffractive optical elements to color separation filters although a brief account is given of digital still cameras. However, the article says nothing about an application of the DOE to phototaking lenses themselves.
With a DOE having a low diffraction efficiency with respect to the design order of diffraction, the intensity of light other than design light (e.g., zero-order light and second-order light when the DOE is designed with first-order light, and hereinafter defined as unnecessary light) becomes too high to obtain good-enough image qualities. Especially in order for the DOE to be used in a wide wavelength range (.lambda.=about 400 nm to 700 nm) wherein phototaking lenses, etc. are used, the diffraction efficiency should be sufficiently high. To improve the diffraction efficiency of the DOE, it is preferable to impart a saw-toothed configuration to the sectional shape of the DOE. In this way, it is theoretically possible to achieve a diffraction efficiency of 100% with respect to one wavelength or one field angle. However, the larger the angle of incidence of a light beam on the diffractive surface, the lower the diffraction efficiency is. For details, see articles "Scalar theory of transmission relief gratings", Optics Communications, Vol. 80, No. 5, 6/307-311 (1991), and "Blazed holographic gratings for polychromatic and miltidirectional incident light", J. Opt. Soc. Am., Vol. 9, No. 7/1196-1199 (1992). Consequently, the zoom lens system according to JP-A 9-211329 cannot be immediately used because the angle of incidence of a light beam on the diffractive surface becomes very large. Also, when the DOE is used in a first lens group in a zoom lens system having a high zoom ratio of about 3 or more, it is required to reduce the angle of incidence of a light beam as much as possible because there is a large change in the angle of incidence of a light beam on the diffractive surface from the wide-angle end to the telephoto end.